Update: Good news. Problem solved without any difficulties. Here's how: I contacted Spencer at his Chapala office. Based on comments on this web and my evaluation of his professionalism (I am a member in good standing of the Law Society of Ontario), I retained him to deal with the matter. He needed an Ontario driver's licence, proof of ownership of the vehicle, proof of insurance, and an Ontario address. After a meeting in his office (of no more than 20 minutes) the Application was in process and we were on our way. Five days later (there was a weekend in between) the Import Permit was received in Canada and on its way to Spencer's office by prepaid Fedex. We picked it up from him 2 days later. Simple, and well worth the fee of about $200 USD. Remember, you pay lawyers for their knowledge. The more they know the simpler the problem looks particularly once it is solved!
Apparently the problem of closed customs offices is not an uncommon one. In future I try and get the permit before leaving Canada. Failing that, I would ask at the actual crossing where the nearest open customs office is and hope to avoid the problem altogether. Still, if you do have a problem don't let people panic you. It can be solved easily.